In the fuel system for a compression ignition internal combustion engine, very high fuel pressures are commonly encountered. For example, in a fuel system of the common rail type, the fuel pressure within the common rail may be around 2000 bar. Clearly, in order to achieve such a pressure, and in order to optimise efficiency it is important to restrict leakage from the various parts of the fuel system, for example the fuel pump and the fuel injectors, to a very low level. In particular, at such high pressures, the fuel pressure within bores formed in the components used in the fuel system may dilate and as a result, the clearance between the walls of the bores and any members slidable within the bores may increase giving rise to increased leakage.